Para-phenylenediamines (PPDs) are known as rubber antioxidants with good performance and widely used in the industry. PPDs are produced by various methods, among which the commonly used are the reductive alkylation method, the phenol-amine condensation method, the hydroxylamine reductive alkylation method, and the quinonimine condensation method. Reductive alkylation of 4-aminodiphenylamine (4-ADPA) and an aliphatic ketone is one of the most important synthetic methods currently used in the industry. For example, N-(1,3-dimethylbutyl)-N′-phenyl-para-phenylenediamine (6PPD) is an important antidegradant for rubbers and functions as an antiozonant, antioxidant, thermoresisting reagent, and flex cracking resisting reagent for rubbers. 6PPD has been widely used as the popularity of the meridian tire arises. 6PPD may be prepared by hydrogenation dehydration of 4-ADPA and methylisobutyl ketone (MIBK) in the presence of a catalyst in a one-step process as follows:

The reductive alkylation may also be a two-step process, during which 4-ADPA and MIBK react through dehydration condensation to form an imine, and the imine is subsequently reduced by hydrogen to 6PPD:

The first step of the reaction may be conducted in the presence of a proton acid catalyst or without a catalyst. The second step must be carried out in the presence of a hydrogenation catalyst with good selectivity.
In the hydrogenation reductive alkylation for preparing the PPD rubber antioxidant, major side reactions include hydrogenation of the raw materials to form the corresponding alcohols, hydrogenolysis of the raw materials or products, hydrogenation of the benzene ring, and tar generated by overheating, etc. The key to obtaining PPD rubber antidegradants with good quality and low cost is to use a catalyst with good activity and selectivity, and can be recycled and reused.
Currently, catalysts for making PPD rubber antioxidants in the industry include copper-based catalyst and the platinum-carbon catalyst. Chinese Patent No. 200610161327.2 discloses a method for preparing a Cu—Zn/Al2O3 catalyst for the reductive alkylation for making 6PPD. The copper-based catalyst costs less, however, the selectivity is not optimal. In the presence of the copper-based catalyst, a large amount of MIBK is hydrogenated and reduced to the corresponding alcohol (MIBA), and the percentage of MIBA at the end of the reaction can reach between 9.3 to 97.4% of the total amount of the MIBK and MIBA. As a result, the raw material is wasted, and the cost is increased. On the other hand, the metal platinum in the conventional platinum-carbon catalyst is very expensive, and the production cost is greatly increased by the bulk use of the catalyst during the industrial production.
Palladium (Pd) has been commonly used as a hydrogenation catalyst in the hydrogenation reduction of the nitro group, carbonyl group, carbon double bond, and carbon-to-nitrogen double bond. However, when palladium-carbon containing catalyst is used as the hydrogenation catalyst in the reductive alkylation for making 6PPD, the raw material and the product have side reactions of hydrogenolysis of their respective C—N bonds so that the selectivity for 6PPD is very low.
To this date, palladium has not been successfully used as an active component in the catalyst for making PPD rubber antioxidants with high conversion rate and high selectivity, because it is still difficult to control the side reactions of C—N bond hydrogenolysis caused by palladium-containing catalysts.